With the great advance of audiovisual systems such as stereo system or television system, equalizers become one of the key components to the audiovisual systems with different quality.
An application example of an equalizer is shown in FIG. 1A, which is an image captured from an application program Winamp of a Microsoft Windows OS (operating system). WINAMP can control audio in the Microsoft Windows OS, wherein a plurality of virtual control buttons are provided for equalization 100, e.g. volume control 102 and sound balance 104. In this example, the equalizer 102 includes ten bands, e.g. 60 Hz, 170 Hz, 310 Hz, 1 KHz, 3 KHz, 6 KHz, 12 KHz, 14 KHz and 16 KHz, independently adjustable through respective virtual dragging buttons 101. The buttons 101 of equalizer 100 shown in FIG. 1A are all tuned at a level of zero decibel (dB), which means the original audio signal is present. FIG. 1B illustrates a frequency response corresponding to the equalizer 100 set at a “0 dB” level, which is ideally flat. In another example, the buttons 101 of equalizer 100 are all set at a level of +12 decibel (dB), as shown in FIG. 2A, in which the desired ideal result is intended to be an absolutely flat frequency response as shown in FIG. 2B. Conventionally, extremely high hardware cost is inevitable in order to achieve the ideally flat frequency response. For example, a complex time-domain circuitry involving inverse Fourier transformation is needed as well as a high-performance microprocessor, e.g. a 32-bit ARM (Advanced RISC Machine) or MIPS (Microprocessor without Interlocked Pipeline Stages) microprocessor. In addition to high cost, it is also a waste of the hardware resource.
Alternatively, the prior art may adopt a low-cost equalizer including several band-pass filters to roughly process several bands by way of a design tool MATLAB. FIG. 3 exemplifies a frequency response of an equalizer including two band-pass filters to process two bands by way of MATLAB. As shown, the two band-pass filters BPA and BPB are roughly centered on frequencies f1 and f2. Accordingly, when all the dragging buttons are tuned to +12 dB, the frequency response only appears flat around the regions near the frequencies f1 and f2. Moreover, serious distortion of audio signals occurs at crossing bands, and thus the quality of the audio signals deteriorates seriously.